What kind of amp are you using and is the amp sound itself vital to your stage sound?
If so, you probably want a speaker-level DI box- this is a DI that goes in between the amp and the speaker and is able to attenuate the signal to the microphone level that the PA is looking for. The Palmer DI mentioned can handle speaker-level signals and has some alright speaker emulation, though they only suggest it for use at a couple hundred watts (I know people have used them successfully with SVTs though). There are also a couple Radial DIs that have speaker emulation and at least one of them can handle speaker signals IIRC.
If your amp itself is basically there to make the sound of your bass louder, then you might be better off with many of the bass preamp/DI boxes listed here that have built-in tone shaping and sometimes speaker emulation. Then you can send the sound guy a signal specifically sculpted for him rather than an amp signal EQ'ed for a speaker. Another possibility in this situation is to use something like the Palmer DI before your amp and after some sort of preamp pedal since it can work at line level as well.
You don't want to put another DI post XLR as you suggested because the XLR on your amp is a DI. The DI on your amp already converts the preamp line level signal into a microphone level, so if you were to send it through another DI the signal might be attenuated even more unless you used an active DI. If you want a post-preamp signal from your amp to sculpt with speaker emulation and don't want to use a speaker level DI, then you'll want to use either a Line Out or FX Loop Send output from your amp. Just make sure the DI you use is configured to accept a Line Level signal.
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